1. Field of the Invention
This invention relates to the preparation of insulating foams having a superior shrinkage resistance, strength and long-term insulating efficiency, and is more particularly concerned with high equivalent weight polyester polyols for the preparation of such foams from compositions containing blowing agents used to replace the conventional fully halogenated chlorofluorocarbons.
2. Description of the Prior Art
It is well known in the manufacture of rigid polyurethane and polyisocyanurate foams to employ chlorofluorocarbons, such as trichlorofluoromethane, as the blowing agent. These chlorofluorocarbon compounds boil or exhibit a significant vapor pressure at ambient temperatures and are volatilized during the exothermic reaction of an isocyanate with an active hydrogen-containing compound, such as a polyol. The expanding gas is entrapped within the reaction mixture and forms an insulating cellular structure. While the foam industry has had good results using the conventional chlorofluorocarbon blowing agents, such as CFC-11, the agents have come under attack in recent years on the ground that they are believed to give rise to environmental problems concerned with ozone depletion in the stratosphere. Accordingly, the search is ongoing for alternative blowing agents with a low ozone depletion factor to replace the conventional ones.
It is believed that hydrochlorofluorocarbons (also known as HCFC's), which are partially halo-substituted hydrocarbons, present less risk than the CFC's. Because the HCFC's contain one or more hydrogen atoms, they more readily dissociate under conditions encountered in the atmosphere, and therefore, less of them would reach the ozone layer of the stratosphere in a form which could cause significant damage. Accordingly, the hydrogen-containing halocarbons are being used as alternatives for CFC-11 in rigid foam applications.
The search for acceptable alternative blowing agents is complicated because of the combination of performance characteristics desired of them. One quite important characteristic is that use of the agents must not cause unacceptable cell shrinkage in the finished foam products. In this regard, it has been found that closed-cell polyisocyanurate foam made with a hydrochlorofluorocarbon such as HCFC-141b and conventional polyols shrinks at low temperatures. It is also desired that the blowing agents have an appropriately low flammability and toxicity. The agents further must not react with the other components of the foam formulation and should be adequately miscible in the foam system. Also, their boiling point, vapor thermal conductivity, capacity to efficiently produce gas and diffusion rate must be appropriate for the formation of highly insulating foams. Finally, the alternative blowing agents should be reasonable in cost.
There still remains a need for a closed cell, rigid foam which has superior properties even though the conventional CFC blowing agents are avoided in its production.